diff --git a/src/lib/search.js b/src/lib/search.js
index de1d634160a..6b24c77cf3c 100644
--- a/src/lib/search.js
+++ b/src/lib/search.js
@@ -71,7 +71,9 @@ exports.sorterDes = function(a, b) { return b - a; };
  * just be off by a rounding error
  * return the distinct values and the minimum difference between any two
  */
-exports.distinctVals = function(valsIn) {
+exports.distinctVals = function(valsIn, opts) {
+    var unitMinDiff = (opts || {}).unitMinDiff;
+
     var vals = valsIn.slice();  // otherwise we sort the original array...
     vals.sort(exports.sorterAsc); // undefined listed in the end - also works on IE11
 
@@ -80,7 +82,9 @@ exports.distinctVals = function(valsIn) {
         if(vals[last] !== BADNUM) break;
     }
 
-    var minDiff = (vals[last] - vals[0]) || 1;
+    var minDiff = 1;
+    if(!unitMinDiff) minDiff = (vals[last] - vals[0]) || 1;
+
     var errDiff = minDiff / (last || 1) / 10000;
     var newVals = [];
     var preV;
diff --git a/src/traces/bar/cross_trace_calc.js b/src/traces/bar/cross_trace_calc.js
index 628bfb1396d..1bb5dda2f81 100644
--- a/src/traces/bar/cross_trace_calc.js
+++ b/src/traces/bar/cross_trace_calc.js
@@ -59,6 +59,9 @@ function crossTraceCalc(gd, plotinfo) {
     }
 
     var opts = {
+        xCat: xa.type === 'category' || xa.type === 'multicategory',
+        yCat: ya.type === 'category' || ya.type === 'multicategory',
+
         mode: fullLayout.barmode,
         norm: fullLayout.barnorm,
         gap: fullLayout.bargap,
@@ -177,6 +180,7 @@ function setGroupPositionsInOverlayMode(pa, sa, calcTraces, opts) {
         var calcTrace = calcTraces[i];
 
         var sieve = new Sieve([calcTrace], {
+            unitMinDiff: opts.xCat || opts.yCat,
             sepNegVal: false,
             overlapNoMerge: !opts.norm
         });
diff --git a/src/traces/bar/sieve.js b/src/traces/bar/sieve.js
index 15052f6f1a9..7cc69f6e81e 100644
--- a/src/traces/bar/sieve.js
+++ b/src/traces/bar/sieve.js
@@ -48,7 +48,10 @@ function Sieve(traces, opts) {
     }
     this.positions = positions;
 
-    var dv = distinctVals(positions);
+    var dv = distinctVals(positions, {
+        unitMinDiff: opts.unitMinDiff
+    });
+
     this.distinctPositions = dv.vals;
     if(dv.vals.length === 1 && width1 !== Infinity) this.minDiff = width1;
     else this.minDiff = Math.min(dv.minDiff, width1);
diff --git a/src/traces/box/cross_trace_calc.js b/src/traces/box/cross_trace_calc.js
index 478dea16abc..72b57856cfb 100644
--- a/src/traces/box/cross_trace_calc.js
+++ b/src/traces/box/cross_trace_calc.js
@@ -68,7 +68,10 @@ function setPositionOffset(traceType, gd, boxList, posAxis) {
     if(!pointList.length) return;
 
     // box plots - update dPos based on multiple traces
-    var boxdv = Lib.distinctVals(pointList);
+    var boxdv = Lib.distinctVals(pointList, {
+        unitMinDiff: posAxis.type === 'category' || posAxis.type === 'multicategory'
+    });
+
     var dPos0 = boxdv.minDiff / 2;
 
     // check for forced minimum dtick
diff --git a/test/jasmine/tests/bar_test.js b/test/jasmine/tests/bar_test.js
index 5b162e897af..8b835f36907 100644
--- a/test/jasmine/tests/bar_test.js
+++ b/test/jasmine/tests/bar_test.js
@@ -969,6 +969,23 @@ describe('Bar.crossTraceCalc (formerly known as setPositions)', function() {
         assertPointField(gd.calcdata, 'b', [[0, 0, 0, 0]]);
     });
 
+    it('should set unit width for categories in overlay mode', function() {
+        var gd = mockBarPlot([{
+            type: 'bar',
+            x: ['a', 'b', 'c'],
+            y: [2, 2, 2]
+        },
+        {
+            type: 'bar',
+            x: ['a', 'c'],
+            y: [1, 1]
+        }], {
+            barmode: 'overlay'
+        });
+
+        expect(gd.calcdata[1][0].t.bardelta).toBe(1);
+    });
+
     describe('should relative-stack bar within the same trace that overlap under barmode=group', function() {
         it('- base case', function() {
             var gd = mockBarPlot([{
diff --git a/test/jasmine/tests/box_test.js b/test/jasmine/tests/box_test.js
index 58ea50c048d..9ea11d6b406 100644
--- a/test/jasmine/tests/box_test.js
+++ b/test/jasmine/tests/box_test.js
@@ -1534,3 +1534,51 @@ describe('Test box calc', function() {
         });
     });
 });
+
+
+describe('Box crossTraceCalc', function() {
+    'use strict';
+
+    function mockBoxPlot(dataWithoutTraceType, layout) {
+        var traceTemplate = { type: 'box' };
+
+        var dataWithTraceType = dataWithoutTraceType.map(function(trace) {
+            return Lib.extendFlat({}, traceTemplate, trace);
+        });
+
+        var gd = {
+            data: dataWithTraceType,
+            layout: layout || {},
+            calcdata: [],
+            _context: {locale: 'en', locales: {}}
+        };
+
+        supplyAllDefaults(gd);
+        Plots.doCalcdata(gd);
+
+        return gd;
+    }
+
+    it('should set unit width for categories in overlay mode', function() {
+        var gd = mockBoxPlot([{
+            y: [1, 2, 3]
+        },
+        {
+            y: [null, null, null]
+        },
+        {
+            y: [null, null, null]
+        },
+        {
+            y: [4, 5, 6]
+        }], {
+            boxgap: 0,
+            xaxis: {
+                range: [-0.5, 3.5],
+                type: 'category'
+            }
+        });
+
+        expect(gd.calcdata[0][0].t.dPos).toBe(0.5);
+    });
+});